To date, eighteen histone deacetylases (HDACs) have been identified in humans. Eleven HDACs (HDAC1-11) are zinc-dependent and seven HDACs, designated sirtuins 1-7, are NAD+-dependent (Parmigiani, R. B. et al. 2008). Aberrant activity of HDACs has been implicated in many disease states, including cancer (Butler, K. V. et al. 2008). When zinc-dependent HDACs are inhibited, the levels of acetylation of certain proteins are elevated, with many resulting physiological effects. Many inhibitors of HDACs have been developed for use against cancers and other disease states. One well-known HDAC inhibitor, suberoylanilide hydroxamic acid (SAHA, Vorinostat), was approved in 2006 for human use following the results of more than 100 human trials against various forms of cancer and is currently in use. Phase I, II and III clinical trials with vorinostat as single therapy and in combination therapy with various anti-cancer agents for hematologic and solid neoplasms are ongoing.
While HDACs are associated with deacetylation of histones in the context of gene expression and chromatin remodeling, there is abundant evidence indicating that not all functions of HDACs are dedicated to deacetylation of histones. Rather, some HDACs have been shown to exert deacetylase activity on proteins other than histones. One such HDAC is HDAC6, a cytoplasmic, microtubule-associated deacetylase, which has been found to regulate microtubule acetylation and chemotactic cell motility (Kawaguchi, Y. et al. 2003).
HDAC6 is predominantly a cytoplasmic, microtubule-associated member of the class IIB family of histone deacetylases. HDAC6 possesses two catalytic domains, a ubiquitin-binding domain and a C-terminal zinc finger domain (Bali, P. et al. 2005). HDAC6 catalyzes deacetylation of cytoplasmic protein substrates, such as α-tubulin, Hsp90, peroxiredoxins, and cortactin (Bali, P. et al. 2005). HDAC6 has also been demonstrated to direct misfolded protein aggregates into aggresomes, which are major repositories formed to manage excessive levels of misfolded and aggregated protein for eventual elimination. Aggresomes are of clinical interest as they are similar to cytoplasmic inclusion bodies commonly observed in neurodegenerative diseases (Gao, Y.-S. et al. 2010).
It has been shown that the C-terminal catalytic domain of HDAC6, the domain responsible for α-tubulin deacetylation, can be inhibited by the small-molecule inhibitor, tubacin (Haggarty, S. J. et al. 2003). Haggarty et al found that the inhibition of HDAC6 with tubacin did not affect the stability of microtubules, but decreased cell motility. Given the dependence of metastasis and angiogenesis on cell movement, increasing the acetylation level of α-tubulin may be an important component to the antimetastatic and antiangiogenic activities of HDAC inhibitors (Haggarty, S. J. et al. 2003).
Heat shock protein 90 (Hsp90) is an important chaperone protein involved in protein folding and is overexpressed in many cancer cell types (Butler, K. V. et al. 2008; Kovacs, J. J. et al. 2005). The disruption of the folding and chaperoning functions of Hsp90 causes its client proteins to be destabilized and eventually degraded. HDAC6 is an attractive target for cancer treatment because acetylated Hsp90 has a reduced ability to perform its chaperoning function (Butler, K. V. et al. 2008; Kovacs, J. J. et al. 2005), with consequent activation of the intrinsic pathway of apoptosis.
In general, for diseases caused by aberrant gene transcription, the most effective treatment would involve targeting only the genes relevant to the disease (Butler, K. V. et al. 2008). In the context of HDAC inhibitor treatment, this would involve inhibiting only those HDAC isoforms relevant to the disease state, thereby minimizing changes not related to the disease, and possibly reducing side effects and toxicity. While SAHA combines efficacy with minimum toxicity, its inhibitory activity is not selective among the known human HDACs.
Marks & Breslow (Marks, P. et al. 2007; Marks, P. et al. 2010) describes the development of HDAC inhibitor voronistat as an anti-cancer drug. HDAC inhibitors have also been identified as a correction for cholesterol and sphingolipid transport defects in human Niemann-Pick type C disease (Munkacsi, A. B. et al. 2011).
In view of the importance of inhibiting only those HDAC isoforms relevant to a disease state, minimizing acetylation of proteins not related to the disease, and reducing side effects and toxicity, new HDAC inhibitors that are selective for specific HDACs are needed.